Mechanism-Dependent Oxygen Tolerance of Mn(bpy)CO₃Br in Electrochemical CO₂ Reduction

CO₂ capture and utilization require the development of highly selective catalysts that convert low-concentration CO₂ into valuable chemicals. Direct air capture (DAC) CO₂ contains up to 10% of oxygen as an impurity, which is a sincere competitor to the electrochemical CO₂ reduction reaction (CO₂RR) due to its favorable thermodynamics. In the best case, the competing O₂ reduction reaction (ORR) leads to lower Faradaic efficiencies of the CO₂RR, and in the worst case, reactive oxygen species (ROS) are generated that lead to catalyst degradation. To circumvent this competing reaction, a catalyst featuring a kinetic advantage for the CO₂RR is required. However, state-of-the-art CO₂ reduction catalysts are rarely explored under dilute or impure conditions. Herein, we investigate the intrinsic reactivity of a homogeneous manganese-based CO₂RR catalyst under DAC-mimicking conditions. Depending on reaction conditions, the catalyst can follow two different mechanisms, which selectively form either CO or formic acid (FA). The oxygen tolerance of the catalyst was found to be dependent on the reaction mechanism: the first step of the CO selective mechanism is CO₂ binding, which is faster than O₂ binding, leading to an intrinsically oxygen-tolerant mechanism; however, the formic acid selective mechanism goes via a manganese-hydride intermediate, which facilitates competing ORR at the metal center.